On the Dynamics of Discharge Processes

Low-order simulations of discharge processes are commonly used in for instance engine modelling. In such simulations the discharge of the engine cylinder is modelled using a discharge coefficient ($C_D$), that represents the effective area of the valve. Experiments to determine $C_D$ are typically performed under steady conditions, assuming that the discharge processes can be regarded as quasi-steady. This assumption is experimentally investigated by comparing $C_D$ obtained under both static and dynamic conditions. The static experiments were performed in a flow bench, with a maximum mass flow of 0.5~kg/s at pressures up to 500~kPa. The dynamic experiments consisted of discharging a pressurised 2~litre fixed volume cylinder with a movable exhaust valve to the atmosphere. The valve opening time was in the range 15-25~ms (equivalent to engines speeds ranging 800-1350~rpm) and the initial pressure was 300-500~kPa. The results show that $C_D$ is a function of the valve opening speed and that static experiments results in elevated values of $C_D$. A measure of ``steadiness" has been defined relating the relative change in flow conditions to the change in system geometry. This measure helps to explain why the process cannot be regarded as quasi-steady.